Delineating the Role of Polyphenol Oxidase in the Darkening of Alkaline Wheat NoodlesJournal of Agricultural and Food Chemistry 54:2378-2385

Fuerst,E.P.,Anderson,J.V.,Morris,C.F.

This study evaluated the effects of inhibitors on polyphenol oxidase (PPO) activity, the effect of the
PPO inhibitor tropolone on noodle darkening, and the correlation of PPO activity with darkening of
alkaline noodles. The PPO inhibitors tropolone and salicylhydroxamic acid (each at 1 íM) reduced
kernel PPO activity by approximately 50% in three hexaploid wheat cultivars but did not inhibit PPO
activity in the two very low PPO cultivars, durum Langdon, and the synthetic hexaploid-derived ID580.
Tropolone (100 íg/g flour) inhibited alkaline noodle darkening (¢L*) by 13-25% in the low PPO
wheat cultivar, ID377s, and by 39-54% in the high PPO wheat cultivar, Klasic. Alkaline noodle
darkening among 502 wheat samples was correlated with kernel PPO activity (r ) 0.64). Results
substantiate the hypothesis that PPO plays a major role in darkening of alkaline noodles. However,
results also indicate that substantial darkening would occur even at zero PPO activity, as measured
in the kernel PPO assay. Therefore, darkening of alkaline noodles is probably due to the cultivarspecific
level of PPO activity and the presence of at least one additional darkening mechanism. Further
investigation is required to identify the phenolic discoloration agent(s) and to determine the potential
roles of non-PPO discoloration mechanisms, both enzymatic and nonenzymatic, in wheat products.